1. Field of Invention
The present invention is directed to methods of assisting in the transfer of images from an intermediate transfer medium to a recording medium. In particular, radiative energy is used to partially cure low viscosity inks to assist in the transfer of images from the intermediate transfer medium to the recording medium during the transfer of the ink from the intermediate transfer medium to the recording medium during the printing process.
2. Description of Related Art
The volume of digital color printing is expected to experience significant growth in the coming years. The color images provided by ink jet printing using solid inks are overwhelmingly preferred in panel studies over other digital imaging systems. There is also a strong case to be made that the total cost of ownership of an ink jet printer will ultimately be cheaper than similar volume electrophotography units. Transfuse plays an important role in piezoelectric ink jet printers by enabling a high quality image to be built up on a rapidly rotating transfer member.
In a typical ink jet printer, the image is applied during four to six rotations with a small translation of the print head in between. This approach simplifies the print head design, while the small movements of the head ensures good droplet registration. The hot melt ink typically used with ink jet printers, e.g., a crystalline wax ink, partially cools on the intermediate transfer member such as a drum or belt and is pressed into the image receiving medium such as paper. This step spreads the image droplet providing a richer color and lower pile height. The low flow of the solid ink prevents show through on the paper.
The current hot melt ink designs work well in transfuse because of the thermally driven changes in rheology. However, the crystalline wax inks do not provide robust images on the paper.
One example of an image transferring method using temperature gradients, e.g., heat, is disclosed in U.S. Pat. No. 6,259,880 to Jia et al.
In particular, the inks currently used in piezoelectric ink jet printers are wax based and are jetted onto a transfuse member, for example, an aluminum drum at temperatures of approximately 130-140° C. The wax based inks are heated to such high temperatures to decrease their viscosity for more efficient jetting onto the transfuse member. The transfuse member is heated to approximately 60° C., so that the wax will cool and thus solidify or crystallize. As the transfuse member rolls over the recording medium, e.g., paper, the image comprised of wax based ink is pressed into the paper.
One problem of the wax based ink is that the inks are soft and scratch easily. Wax based inks generally crystallize at temperatures greater than room temperature. Therefore, the wax based ink that has been transferred to the recording medium is essentially as hard as it will get.
Another problem of using wax based inks that crystallize is that the use of a low viscosity oil, such as silicon oil, on an intermediate transfer member is necessary. The oil is used to release the ink located on the transfer member so the image can be pulled off the transfer member onto the recording medium, e.g., paper. Without the oil, part of the ink would remain on the transfer member. However, a small portion of the oil will be transferred onto the recording medium. Any oil transferred onto the recording medium is quickly diffused. However, until the oil has been diffused it is not possible to write on the recording medium.
Thus, curing by photoinitiation of reactive inks is of interest because ink cured in such a manner provides tough, permanent images on paper. These photocurable inks can be designed to have low viscosity and avoid the need to heat the print head beyond what may be required for thermal stability.
However, a low viscosity ink is difficult to transfuse because the ink droplets may coalesce during transfer member rotation and additionally the low viscosity ink will show through the paper. There are two shortcomings with low viscosity inks: (1) coalescence may occur, for example, a row of five closely spaced drops might merge into a single blob, and (2) show through occurs when the low viscosity ink wicks through the paper to become noticeable on the opposite side rather than sitting on the surface it was printed on.
Further, low viscosity ink, such as radiation curable ink, does not hold its location well on the transfuse member and is therefore not currently the preferred ink in an ink jet printer such as a piezoelectric printer or an acoustic ink jet printer.
The drops of the low viscosity ink tend to run together when transferred onto the recording medium. Also, the final image may be hazy, feathered, and may show through on the other side of the recording medium.